Impact of Active Control Turbocharging on the Fuel Economy and Emissions of a Light-Duty Reactivity Controlled Compression Ignition Engine: A Simulation Study

Anand Nageswaran Bharath,Christopher J Rutland,Rolf D Reitz

doi:10.3389/fmech.2021.610891

Abstract

While forced induction strategies such as turbocharging can increase the power output and extend the load limit of engines operating on low temperature combustion strategies such as reactivity controlled compression ignition, the low exhaust enthalpy prevalent in these strategies requires the use of high backpressures to attain high turbocharger efficiencies, leading to high pumping losses and in turn poor fuel economy. Hence, there is a need to improve the exhaust energy utilization by the turbocharger such that the negative effects of the high backpressure requirements are offset. One turbocharger operating strategy that has the potential to enhance exhaust enthalpy conversion by the turbine is active control turbocharging (ACT), in which the rack position of a variable geometry turbocharger (VGT) is actuated using a continuously varying sinusoidal signal whose frequency is proportional to engine speed. In this study, the impact of ACT on turbocharger performance and fuel economy of a light-duty reactivity controlled compression ignition engine equipped with a VGT is investigated through coupled GT-POWER/KIVA-3V simulations at a medium-load cruise operating condition. A design of experiments study was executed in which the rack position amplitude and phase angle were independently varied, and the turbine efficiency, compressor efficiency, crankshaft torque, and brake specific fuel consumption were tracked for each run. The results show that ACT operation significantly increased the torque output while improving fuel economy over baseline VGT operation, but the range of actuation signal amplitude ratio was limited to 40% of the maximum amplitude possible due to peak cylinder pressure and peak pressure rise rate constraints. It is also shown that the impact of signal phase angle on turbocharger efficiency and overall system performance is not as significant as the amplitude ratio. The best fuel economy improvement over the baseline VGT operation at cruise conditions was observed at 40% amplitude ratio and 0° phase angle, and this value was 2.8%.

Highlights

The transportation sector is one of the largest contributors to greenhouse gas emissions, accounting for 29% of total emissions in the United States in 2017 (U.S EPA, 2019); this number is expected to increase in the future due to the increased demand for travel
This study presents simulation results from a one-dimensional gas dynamics model coupled to an in-cylinder computational fluid dynamics (CFD) model on the behavior of a light-duty reactivity controlled compression ignition (RCCI) engine system equipped with an active control turbocharger
Going from 40% amplitude ratio with phase angle at 0° to 60% amplitude ratio at the same phase angle causes the in-cylinder trapped mass to increase by 6.96%, which causes the peak cylinder pressures (PCP) to increase by 10.8% from 149.3 to 165.4 Bar

Summary

Introduction

The transportation sector is one of the largest contributors to greenhouse gas emissions, accounting for 29% of total emissions in the United States in 2017 (U.S EPA, 2019); this number is expected to increase in the future due to the increased demand for travel. To mitigate the impact of these emissions, government regulations for vehicular greenhouse gas limits are becoming more stringent, calling for significant reductions in tailpipe carbon dioxide (CO2) emissions and radical improvements in fuel economy through the use of novel engine technologies. One such technology that has attracted significant attention is low temperature combustion (LTC). To allow smooth high load LTC operation, it is necessary to use forced induction strategies such as turbocharging and supercharging in combination with exhaust gas recirculation (EGR) so as to increase the working fluid quantity while diluting the fuel-charge mixture for a more controlled heat release at higher loads (Dec and Yang, 2010). Due to the low exhaust gas enthalpy that is prevalent in LTC strategies, it is necessary to use small turbochargers for efficient exhaust energy conversion, which leads to high engine backpressures that in turn contribute to large pumping losses and poor fuel economy (Olsson et al, 2001)

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